I like to sit pretty far back on the saddle, and traditional touring bags tend to bump into my thighs if mounted in the standard manner. I have dealt with this by making a standoff from a piece of 2x3. I bored a 1 1/8 hole edgewise through a scrap 2x3, then cut it apart down the middle of the hole. This creates a nice semicircular fit against the seatpost. The stock Brooks seatpost strap wasn't long enough to reach around this, so I cut an old toe strap down to size. A staple keeps the spacer block and strap together when the bag is off the bike.